As is well known, a servo motor system is able to precisely control the rotation speed of a servo motor, and has the fast response to acceleration, deceleration and reversion. Due to the precise position control capability and the speed control capability, the servo motor system has been widely used in various industrial automation industries and precision machining fields such as mechanical arms or mechanical work platforms.
FIG. 1A is a schematic functional block diagram illustrating the architecture of a conventional servo motor system. As shown in FIG. 1A, the servo motor system comprises a command device 110, a micro controller 120, a servo motor 130, and an optical encoder 140.
In response to the user's operation, the command device 110 generates a command pulse for controlling a rotation speed and a rotation direction of the servo motor 130. Moreover, according to the rotation speed and the rotation direction of the servo motor 130, the optical encoder 140 generates a feedback pulse to the micro controller 120. Moreover, according to the command pulse and the feedback pulse, the micro controller 120 generates a driving pulse to the servo motor 130.
By the optical encoder 140, a displacement amount of a rotating shaft of the servo motor 130 is transformed into three photoelectronic signals A, B and Z. The photoelectronic signals A, B and Z are transmitted to the micro controller 120. According to the feedback pulse from the optical encoder 140, the micro controller 120 may realize the rotation speed, the rotation direction and the position of the servo motor 130.
For example, the optical encoder 140 is a rotary optical encoder. The optical encoder 140 comprises a light source 142, a photo detector 146, and a disk 148. The disk 148 is coupled to the rotating shaft of the servo motor 130. In addition, the disk 148 is rotated with the servo motor 130. Moreover, after a light beam emitted by the light source 142 passes through gratings of the disk 148, the light beam is received by the photo detector 146. According to the shapes of the gratings of the disk 148, the photo detector 146 generates the photoelectronic signals A, B and Z. During acceleration or deceleration of the servo motor 130, the frequencies and phases of the photoelectronic signals A, B and Z are subjected to changes. In other words, the photoelectronic signals A, B and Z are time variant signals.
Please refer to FIG. 1A again. The first photoelectronic signal A contains an A+ sensing current and an A− sensing current. The second photoelectronic signal B contains a B+ sensing current and a B− sensing current. The third photoelectronic signal Z contains a Z+ sensing current and a Z− sensing current. The way of forming the first photoelectronic signal A and the way of forming the second photoelectronic signal B and the way of forming the third photoelectronic signal Z are identical. Hereinafter, only the way of forming the first photoelectronic signal A will be illustrated, but the way of forming the second photoelectronic signal B and the way of forming the third photoelectronic signal Z will not be redundantly described herein.
FIG. 1B is a schematic timing waveform diagram illustrating the A+ sensing current and the A− sensing current of the first photoelectronic signal A. In the photo detector 146 of the optical encoder 140, two sensing elements are used to generate the A+ sensing current and the A− sensing current. The A+ sensing current and the A− sensing current are photo sensing currents. Obviously, the phase difference between the A+ sensing current and the A− sensing current is 180 degrees.
Moreover, due to the difference between the two sensing elements, the A+ sensing current and the A− sensing current may be suffered from different DC offsets. As shown in FIG. 1B, the A+ sensing current fluctuates between −1.2 μA and −4.2 μA, and the DC offset of the A+ sensing current is −2.7 μA. Moreover, the A− sensing current fluctuates between −1.5 μA and −5.7 μA, and the DC offset of the A− sensing current is −3.6 μA.
As known, the first photoelectronic signal A to be processed by the micro controller 120 should have no DC offset. Consequently, the micro controller 120 should be equipped with a DC offset cancellation circuit to generate the first photoelectronic signal A without the DC offset.
FIG. 2 schematically illustrates a conventional DC offset cancellation circuit. As shown in FIG. 2, the DC offset cancellation circuit 20 comprises a first current-to-voltage converter (I/V) 210, a second current-to-voltage converter 220, a first voltage amplifier 230, a second voltage amplifier 240, a differential amplifier 250, an analog-to-digital converter (ADC) 260, and a digital processing unit (DSP) 270.
After a first sensing unit 202 issues the A+ sensing current to the first current-to-voltage converter 210, the first current-to-voltage converter 210 generates an A+ voltage signal (VA+). After the A+ voltage signal (VA+) is inputted into the first voltage amplifier 230, an amplified A+ voltage signal (GVA+) is generated.
After a second sensing unit 204 issues the A− sensing current to the second current-to-voltage converter 220, the second current-to-voltage converter 220 generates an A− voltage signal (VA−). After the A− voltage signal (VA−) is inputted into the second voltage amplifier 240, an amplified A− voltage signal (GVA−) is generated.
After the amplified A+ voltage signal (GVA+) is received by a positive input end of the differential amplifier 250 and the amplified A− voltage signal (GVA−) is received by a negative input end of the differential amplifier 250, the differential amplifier 250 generates the first photoelectronic signal A. After the first photoelectronic signal A is received by the analog-to-digital converter 260, a digitized first photoelectronic signal Ad is generated. After the digitized first photoelectronic signal Ad is received by the digital processing unit 270, a control signal C is generated.
From the above discussions, the DC offset cancellation circuit 20 of FIG. 2 uses the digital processing unit 270 to process the digitized first photoelectronic signal Ad and generate the control signal C according to the DC offset. According to the control signal C, the gain values of the first voltage amplifier 230, the second voltage amplifier 240 and the differential amplifier 250 are correspondingly adjusted. Consequently, the first photoelectronic signal A without the DC offset is outputted from the differential amplifier 250.
FIG. 3 schematically illustrates another conventional DC offset cancellation circuit. As shown in FIG. 3, the DC offset cancellation circuit 30 comprises a first current-to-voltage converter (I/V) 310, a second current-to-voltage converter 320, a differential amplifier 350, an analog-to-digital converter (ADC) 360, a digital processing unit (DSP) 370, and a digital-to-analog converter (DAC) 380.
After a first sensing unit 202 issues the A+ sensing current to the first current-to-voltage converter 310, the first current-to-voltage converter 310 generates an A+ voltage signal (VA+). After a second sensing unit 204 issues the A− sensing current to the second current-to-voltage converter 320, the second current-to-voltage converter 320 generates an A− voltage signal (VA−).
After the A+ voltage signal (VA+) is received by a positive input end of the differential amplifier 350 and the A− voltage signal (VA−) is received by a negative input end of the differential amplifier 350, the differential amplifier 350 generates the first photoelectronic signal A. After the first photoelectronic signal A is received by the analog-to-digital converter 360, a digitized first photoelectronic signal Ad is generated.
After the digitized first photoelectronic signal Ad is received by the digital processing unit 370, a control signal C is generated. According to the control signal C, the gain value of the differential amplifier 350 is correspondingly adjusted.
Moreover, the digital processing unit 370 may process the digitized first photoelectronic signal Ad and generate a DC value (DCd) to the digital-to-analog converter 380 according to the DC offset. Consequently, the digital-to-analog converter 380 generates a DC voltage Vdc.
From the above discussions, the DC offset cancellation circuit 30 of FIG. 3 uses the digital-to-analog converter 380 to generate the DC voltage Vdc in order to cancel the DC offset of the first photoelectronic signal A. Consequently, the first photoelectronic signal A without the DC offset is outputted from the differential amplifier 350.